1. Field of the Invention
The present invention relates to an elastic wave device including a thin film of a piezoelectric single crystal and a method for manufacturing the elastic wave device.
2. Description of the Related Art
In recent years, many piezoelectric devices each including a thin film of a piezoelectric single crystal have been developed. In the piezoelectric device including a piezoelectric thin film as described above, a support member supporting the piezoelectric thin film is necessary in practical use. As disclosed in Japanese Unexamined Patent Application Publication No. 2007-228319 and Japanese Unexamined Patent Application Publication No. 2003-17967, the support member as described above is provided on one primary surface of the piezoelectric thin film.
In addition, as one of methods for forming a composite piezoelectric substrate formed of the piezoelectric thin film and the support member as described above, for example, a smart cut method was used. In the smart cut method, an ion-implanted layer is formed by performing ion implantation in one primary surface of a piezoelectric substrate having a bondable thickness. Next, a support member separately formed is bonded to the primary surface at an ion-implanted layer side of the piezoelectric substrate in which the ion-implanted layer is formed, for example, by using activation bonding or affinity bonding. Subsequently, the piezoelectric thin film is peeled away from the piezoelectric substrate by heating the ion-implanted layer.
Incidentally, as a method for bonding a support member to a substrate which forms a thin film as described above is by peeling, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-118079, although not for a composite piezoelectric substrate, the structure has been proposed in which an elastic body is provided between a single crystal silicon substrate and a support member.
However, when the piezoelectric substrate and the support member are directly bonded to each other as disclosed in Japanese Unexamined Patent Application Publication No. 2007-228319 and Japanese Unexamined Patent Application Publication No. 2003-17967, the difference in coefficient of linear expansion between the piezoelectric substrates and the support member cannot be ignored, and hence the degree of freedom of selecting a material of the support member will be remarkably limited. In addition, in order not to apply an unnecessary stress to the piezoelectric substrate during bonding, the bonding conditions must be strictly set, for example, so as to avoid irregularities greater than a predetermined level and to prevent particles from being present on a bonding surface. Hence, concomitant with an increase in process load, the process control becomes difficult.
On the other hand, as disclosed in Japanese Unexamined Patent Application Publication No. 2008-118079, when the elastic body is provided between the semiconductor substrate and the support member, various troubles as described above generated when the piezoelectric substrate and the support member are bonded to each other can be suppressed. However, since an elastic body having a low elastic modulus is bonded to the piezoelectric substrate, damping is unfavorably generated, and the performance as the piezoelectric device is degraded. In particular, in the case of an elastic wave device using a piezoelectric thin film, the damping as described above must be avoided since the performance is seriously degraded thereby.